I have an 87 GTA, 5.7l auto. The car runs fine except that once I get above around 40mph, and then decelerate, it will idle at 1200 rpm until about 3 seconds after I come to a complete stop. If I put it in neutral, it will stay at 1200. If I don't get it above about 1200, it will idle back down to 600 or so normally.
I have reset the IAC and the TPS, switched ecms, and checked the throttle linkage.
The only other thing I can add is that I replaced the trans a while back but I can't remember if this problem started right after that or not.
What do you think might be causing this problem?

 

Have you tried replacing the IAC? I had a real bad problem with idle. Like you said, it would stay up (mine was around 1500-2000). So I replaced the TPS first, and that didn't solve the problem. So then I replaced the IAC. Followed the instructions to reset it, and now the car idles just fine.

Thats my story, may work for you.

 

Clean your idle passages, pay particular attention to the bore where the IAC pintle goes into the throttle body housing.